Something interesting so far, neither Springman nor Rex were stated to be not included in the roster due to being assist trophies or Mii costumes already, but rather unrelated things (Arms not having one singular main character/minimin being the director's favorite and Rex and Pyra duo were scrapped due to hardware/software limitations). So as far as we know, Both already being a Mii Costume or assist trophy doesn't seem to be a main factor if a character doesn't get in. Of course that doesn't mean it's likely a character who already is one or the other will get it, but from everything we've seen and been told, it isn't a factors against those characters.
So yeah, holding out some hope for Waluigi, as well as some other assist/costume upgrades
Waluigi will be the final character.
There's just a lot of things that are going for him, it can't be a coincidence.
The mere fact that Nintendo would choose Min Min and Pyra/Mythra, two (well actually three) first-party characters that were supposed represent their respective games in base game Ultimate but couldn't due to time constraints, is actually favorable to Waluigi's chances.
But there's also a catch, they didn't simply go over Spring Man because he was already an Assist Trophy.
Other than the main reason for choosing Min Min which was the ARMS director personal wishes, Spring Man (and no offense to his fans) is incredibly boring when compared to Min Min.
And Min Min is more popular according results from an official tournament/event that took place for ARMS.
Now you tell me what Nintendo character has exploded in popularity in the last 3 years, and no Bowsette doesn't count.
Something tells me that even though Nintendo likes to dunk on Waluigi from time to time, they wouldn't do it here, especially when they already picked two (again they're three) characters that were popular requests for Smash Ultimate, and currently Waluigi is the ultimate (Pun intended) first-party request for Smash.
Where's the Luigi's Mansion 3 Spirits ???
That game sold over 5 mil.
I bet they're holding onto them for when Waluigi arrives on October.
Let's not forget that Nintendo opted to add Waluigi to Mario Kart Tour on October in the Halloween event, and grouped him with none other than the Luigi's Mansion version of King Boo.